This invention relates to the field of metal forming and, more particularly, to the forming of metals which exhibit superplastic characteristics.
Superplasticity is the characteristic demonstrated by certain metals which exhibit extremely high plasticity in that they develop unusually high tensile elongations with minimum necking when deformed within limited temperature and strain rate ranges. The methods, applicable to the teachings of this invention, used to form the superplastic materials capitalize on these characteristic and typically employ gas pressure to form sheet material into or against a configurational die in order to form the part. diffusion bonding is frequently associated with the process. U.S. Pat. No. 3,340,101 to D. S. Fields, Jr. et al, U.S. Pat. No. 4,117,970 to Hamilton et al, U.S. Pat. No. 4,233,829 also to Hamilton et al, and U.S. Pat. No. 4,217,397 to Hayase et al are all basic patents, with various degrees of complexity, relating to superplastic forming. All of these references teach a process which attempts to control stress, and thereby strain, by controlling the pressure in the forming process versus time.
One exception to controlling forming rates by controlling pressure versus time is taught in U.S. Pat. No. 4,708,008 to Yasui et al. Yasui is also the named inventor herein with the same assignee as the reference. The Yasiui reference teaches measuring and controlling the volume displaced by the blank being formed so as to measure total strain or surface area increase of the blank.
U.S. Pat. No. 4,489,579 to Daime et al also control the process by controlling pressure versus time but also teaches additional devices for monitoring the forming rate by providing a tube which penetrates the die and engages a portion of the blank to be formed. As the blank is formed, the tube advances through the die directly as that portion of the blank is formed. Means are also provided to produce a signal at predetermined amounts of advancement of the tube and, further, electrical contacts are provided at recess angles of the die and the switch is closed when the blank being formed, it provides for monitoring the forming step which allows the operator to evaluate the development process of the part. However, it is not very practical to have a sliding tue probe with the associated geometric disturbance at the contact point nor is it practical to provide electrical instrumentation in this harsh environment.
Keep in mind that excessive strain rates cause rupture and must be avoided in the forming process. In order to understand excessive strain rates it is necessary to understand the relationship between the variables in superplastic forming which are represented by the classic equation: EQU .delta.-K.epsilon.m
where m is the strain rate sensitivity, .delta. is stress, .epsilon. is strain rate, and K is a constant.
In the absence of strain hardening, the higher the value of m, the higher the tensile elongation. Solving the classic equation for m, ##EQU1## In addition to strain rate, the value of m is also a function of temperature and microstructure of the material. The uniformity of the thinning under biaxial stress conditions also correlates with the value of m. For maximum deformation stability, superplastic forming is optimally performed at or near the strain rate the at produces the maximum allowable strain rate sensitivity. However, because the strain rate sensitivity, m, varies with temperature as well as stress and microstructure, m is, as a practical matter, constantly varying during the process.
Furthermore, the strain rate varies at different instances of time on different portions of the formation inasmuch as stress levels are non uniform. The more complex the part, the more variation there is, and, therefore, strain rate differs over the various elements of the formation. Since strain rate, stress, temperature and microstructure are all interdependent and varying during the process, the relationship is theoretical. As a practical matter, there is no predictable relationship which can be controlled so as to form all portions of complex parts at the optimum strain rate sensitivity and therefore the optimum strain rates. However, the artisan can plot strain rate sensitivity (m) against strain rate (.epsilon.) and stress (.rho.) against strain rate (.epsilon.) and establish the best compromise ranges to be caused as guides. Those skilled in the rt must then select and control those portions of the formation which are more critical to successful forming while maintaining all other portions at the best or less than the best strain rates which necessarily becomes the overall optimum rate.
This is further complicated for deep forming, which requires forming pressure reduction due to the higher thinning rate of the material, if during the forming process, the blank may not be exactly where it is thought to e at any given time int the forming process. For example, FIG. 3 shows a typical pressure versus time curve for forming a cylinder having a bottom. The discontinuity in the ideal pressure and mass flow curves at approximately 58% of the total time is where the bottom portion of the cylinder being formed first touches the fixture. Obviously greater stress is required to form the balance of the specimen. What is critical is to pick the point where the slop changes and the two slopes could be a straight line or a linear flow rate. However, the pressure is actually reducing between the 20% and the 58% points on the time abscissa. In other words, if a simple cylinder was being formed as shown in the Figures and the artissman has determined by an analysis, as discussed above, that after a period of time t.sub.1 (58% time in FIG. 3) the blank has formed to the extent that the spherical portion has touched the upper portion of the configurational die the normal process would demand an increase in forming pressure to from the recess corner between the wall and bottom of the cylinder. If for some reason the spherical portion of the forming blank had not, as anticipated, reached the fixture the programmed pressure increase would cause an excessive strain rate as the specimen should be still forming at the lower pressure. Since the process is being pressure controlled, the system will respond and accelerate the strain rate until rupture occurs.
By controlling the process by either volume or pressure alone only one of the variables in Boyle's Law ##EQU2## (where P, V, and T represent pressure, volume, and temperature, respectively) is used to control the process. Even though Daime et al teaches an aid to measure critical displacement the reference still teaches controlling the process by controlling pressure. Further, as previously indicated, the sliding, protruding tubes from the fixture in the forming environment is not practical, particularly where the part is complex and would require many protruding tubes.
It is an object of the present invention to provide a method for controlling superplastic forming processes which is self-correcting in that if the part strain rate increase, the forming rate self-adjusts because the resulting increase in volume of the specimen a being formed acts to reduce the forming pressure.
It is a further object of this invention to control all the variables in Boyle's Law during the forming process.